With the increase of worldwide production of biodiesel, a large number of studies have been carried out with the objective of absorbing the excess glycerine available on the market, about 10% of the total production of biodiesel, which reduces its economic value, and at the same time generating new products with higher added value.
In a first phase, products such as 1,2-propanediol can be obtained from glycerine. This raw material, which is also available in abundance, has become important in obtaining chemical products which are of great interest currently.
Lactic acid is an important raw material for the chemical industry currently and is widely consumed in different industrial sectors. It has application in the foods industry in the fabrication of various products, for example, such as yoghurt and cheese. It is also used in the cosmetics industry as a wetting agent; in the textile industry as a mordant and for curing of leather.
The modern petrochemical industry, which is now facing the challenge of seeking sustainable means to provide green products of low environmental impact, is looking to lactic acid as one of the important raw materials for the production of biodegradable materials, particularly in the obtaining of poly(lactic acid)—PLA.
PLA is a biodegradable, compostable and biocompatible polymer with a vast range of applications. Besides its well established use in the medical area, its use in plastic packaging has received increasing attention, which in being considered a green polymer, minimises impacts on the environment.
Lactic acid can be produced by different means, and is an example of a classic process of production based on the reaction between acetaldehyde and cyanic acid followed by the stage of hydrolysis with sulphuric acid or by the reaction of carbon monoxide with formaldehyde in water at high pressures using hydrofluoric acid as a catalyst. These means of synthesis use highly hazardous and toxic products, take place in a homogeneous medium, generate polluting liquid waste with a high environmental impact and entail processes with a high cost. As a consequence, lactic acid is fabricated commercially these days by fermentation of sugars.
However, the fermentation or biochemical processes have disadvantages in taking a long time and being of quite high cost, as they make use of complex micro-organisms which require special care for their growth, development and maintenance, involving large volume equipment which significantly increases the cost of production.
Lactic acid can also be obtained by the chemical transformation of other sources. The academic technical literature for example shows a process where glycerine is used in a reaction which takes place in an alkaline medium in homogeneous phase and under hydrothermic conditions [H. Kishida, F. Jin, Z. Zhou, T. Moriya, H. Enomoto, Chem. Lett. 34 (2005) 1560-1561]. Although the yields of lactic acid reach about 90%, the reactions take place at very high temperatures, at around 300° C., and considerably high pressures. Furthermore, the final product obtained is the derived salt of lactic acid, lactate, as a function of the addition of alkaline solution to the reaction medium. Thus, the stage of hydrolysis with an inorganic acid at the end of the synthesis is required to obtain the free lactic acid. These conditions entail processes of high cost as well as high energy cost, also requiring equipment constructed with special materials in order to avoid corrosion of the reactors due to the high concentration of hydroxide.
Selective oxidation reactions of the primary hydroxyl of the 1,2-propanediol to lactic acid in an aqueous medium using a heterogeneous catalyst have also been reported in the literature [M. Hong, N. Xin, C. JiaYing, C. Chen, G. Jin, M. Hong, X. Jie, Sci. China Chem., 53 (2010) 1497-1501]. In these processes, the authors describe the use of gold catalysts supported in Mg(OH)2, which lead to 94.4% conversion of 1,2-propanediol, 89.3% selectivity of lactic acid, resulting in 84.3% yield of acid after 6 hours of reaction. However, in this process there is also the need to add a solution of NaOH, thus leading to lactic acid salt as the final product. The authors describe the use of a molar ratio between NaOH and 1,2-propanediol equal to 2 and the process takes place at quite a high temperature (600° C.) and pressures above atmospheric pressure, with the use of partial pressure of O2 of the order of 3 bar being described.
Other studies in the scientific literature also describe the oxidation of diols, focusing essentially on the application of metallic catalysts based on gold at temperatures of the order of 70° C. to 90° C. at pressures of between 2 bar and 3 bar of pure oxygen and the addition of a base to maintain a constant pH. The yields of lactic acid are always in the range between 5% and 64% and this performance may be the reason for the comparison with systems based on platinum or palladium supported on activated charcoal [S. Demirel, P. Jern, M. Lucas, P. Claus, Catal. Today 122 (2007) 292-300; L. Prati, M. Rossi, J. Catal. 176 (1998) 552-560; C. Bianchi, F. Porta, L. Prati, M. Rossi, Top. Catal. 13 (2000) 231-236].
The document of patent CN 101225041 concerns a process which uses gold catalysts on different supports and the addition of NaOH or KOH to control the pH at 10. It is possible to obtain lactic acid, although with very low yields at the specified conditions, varying in the range between 9.7% and 32% and reaching 81% at the highest conversion of glycerine. Lactic acid is not obtained directly in free form, with subsequent stages of hydrolysis being necessary for the production of the acid, such as fermentative processes which generat solid residues to be discarded.
The document of patent EP 2 184 270 concerns a process for production of lactic acid from glyceraldehyde, using zeolite beta acid as a catalyst containing tin in the structure. The process also allows the corresponding esters to be obtained by using a suitable solvent. Thus, to obtain methyl lactate, methanol is used as a solvent. A selectivity of 16% is obtained in methyl lactate from glyceraldehyde in the presence of Sn-BETA catalyst at 100° C. over 20 hours at a pressure of argon of 20 bar and using methanol as the solvent. In these processes, no base or alkaline solution is added to control the pH, however the fact that the reaction is processed in an inert atmosphere (argon) at quite high pressures must be considered.
In a recent article [A. Tsuji, K. T. V. Rao, S, Nishimura, A. Takagaki, K. Ebitani, ChemSusChem, 4 (2011) 542-548], the use of reactions of selective oxidation of the primary hydroxyl of the glycerol or of 1,2-propanediol to glyceric acid and lactic acid is reported respectively in an aqueous medium making use of mild temperature conditions and flow of pure oxygen, without the addition of a base. To this end, Pt/hydrotalcite catalysts with different Mg/Al ratios are used but with extremely high levels of platinum, of more than 35% by weight. In the case of oxidation of the 1,2-propanediol, after 6 hours of reaction, selectivity towards lactic acid of 70% is obtained and conversion of the diol of around 65%, which gives a yield in lactic acid of 45.5%.
In most of the state of the art processes using a heterogeneous catalyst, the pH of the reaction medium has to be controlled by the continuous addition of an alkaline solution. This operation does not allow the direct production of lactic acid, but rather of its respective salt, a lactate.
This fact is an important disadvantage, as like the fermentative processes, the lactate that is produced has to be hydrolised by the addition of an inorganic acid in a subsequent operation. At this stage, large amounts of solid residue are generated, and this has to be separated and discarded, as in the biological processes. Besides the environmental impact, the need of various stages to obtain the lactic acid increases the complexity of the process, and consequently its cost.
As can be seen, on the basis of the description of this invention that follows, lactic acid is obtained in a single stage, eliminating the step of addition of alkaline solutions, with no need to control the pH, and the elimination of solid residue formation and consequently no additional separation, treatment or discarding of solid residue is necessary. These various advantages afforded entail a reduction of the costs of the process and mitigate the environmental impact, establishing an environmentally friendly process.